Classic Oddball

Not too much to say on me I am new to this site and my body has always been stubborn. I don't tend to eat many calories on a normal basis ( even when not on a diet/ lifestyle change ) so the calories I consume are always on the 700-900 mark have been for ages. I have always been a big meat eater. i run borderline anemic and need the iron. My son was born and my body went into freak out mode. I had 30-40 lbs of water that were in my cells b/c of pregnancy decide to fill my lungs and heart cavity. Dr. was an idiot and sent me home. after and emergency room visit and an echo I was given heart medicine to help my heart recover, well my body said what the "F" and started lowering my thyroid output. which of coarse effected everything else, insulin, mood energy, my poor son and husband had to deal with me some days I could barely walk and even then I would have to stop and catch my breath as you can imagine my fat stayed and sagged. well My heart and thyroid have healed and Ive got some energy so Im going for it 50lbs Ill be happy more than that Ill be ecstatic ( I just don't want to get below 175 then id be starved) but what I want to know is if you maintain a calorie intake of 700-900 with exercise three times a week why would that not freak out the system at first then start dropping lbs when the body realizes its not going to die. I thought all weight loss theory started with a hold onto fat period then hit a ok I dont need this food period till you got down to were you wanted?